578 GENERATION. SECT. XXXIX. 5, 



that furface had been white like the reft of his 

 feathers. 



There is a ftill more wonderful thing concerning 

 thefe colours adapted to the piirpofe of conceal- 

 ment ; which is, that the eggs of birds are fo co- 

 l6ured as to refemble the colour of the adjacent 

 objects and their interftices. The eggs of hedge- 

 birds are greenifh with dark fpots ; thofe of crows 

 and magpies, which are feen from beneath through 

 wicker nefts> are white with dark fpots; and thofe 

 of larks and partridges are ruflet or brown, like 

 tiheir nefts or fituations. 



A thing ftill more aftonifhing is, that many ani- 

 mals in countries covered with fnow become white 

 in winter, and are faid to change their colour again 

 in the warmer months, as bears, hares, and par- 

 tridges. Our domefticated animals lofe their natu- 

 ral colours, and break into great variety, as horfes, 

 dogs, pigeons. The final caufe of thefe colours is 

 eafily underftood, as they ferve fom'e purpofes of 

 the animal, but the efficient caufe would feem al- 

 triQft beyond conjecture. 



Firft, the choroid coat of the eye, on which the 

 femitranfparent retina is expanded, is of different 

 colour in different animals ; in thofe which feed on 

 grafs it is green ; from hence there wbuld appear 

 fome connexion between the colour of the choroid 

 coat and of that conftantly painted on the retina by 

 the green grafs. Now, wheri the ground becomes 

 covered with fnow,' it would feem, that that action 

 of the retina, which is called whitenefs, being con- 

 ftantly excited in the' eye, maybe gradually imi- 

 tated by the extremities of the nerves of touch, or 

 rete m'ucofum of the fkin. ; And if it be fuppofed, 

 that the a&ion of the retina in 'producing the per- 

 fception of any colour confifts in fo difpofing its 

 own fibres or furface, as to refl^cl thofe coloured 

 rays only, and tranfmit the others like foap-bub- 



bles; 



